In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
EP 1 624 451 discloses a holographic storage system with a coaxial arrangement, where a plurality of reference beams are arranged around the object beam. According to this solution the object beam and the reference beams are coupled in and out at the object plane and the image plane, respectively. This arrangement is a so-called split aperture arrangement, because the aperture of the Fourier objective is split into an object part and a reference part. The arrangement has the advantage of being circularly symmetric. The selectivity and also the interhologram crosstalk are the same in every direction in the plane of the holograms. However, half of the aperture of the Fourier objective is used for the reference beams. This means that the capacity of a single hologram of the split aperture system is only half of the capacity of a common aperture arrangement. In addition, a total overlap of the object beam and the reference beams occurs only in the Fourier plane. The overlap is only partial within a 100-200 μm thick layer in the holographic storage medium. This value depends on the diameter of the hologram and the numerical aperture of the Fourier objective. Starting at a distance of about 200-400 μm from the Fourier plane there is no overlap at all.
In WO2006/003077 a 12 f reflection type coaxial holographic storage arrangement with three confocally arranged Fourier planes is shown. In this arrangement the object beam and the reference beams are coupled in and out at the first and third Fourier planes, respectively. The reference beams are small spots in these planes. More precisely, they form diffraction patterns, similar to the Airy pattern. This arrangement is a so-called common aperture arrangement, because at the object plane and the image plane the object beam and the reference beams fill out the same area of the aperture. The beams fill out the entire aperture of the objectives. The disclosed arrangement allows to apply shift multiplexing, reference scanning multiplexing, phase coded multiplexing, or a combination of these multiplexing schemes. The reference beams are a pair (or pairs of) half cone shaped beams. The tips of the pair or pairs of half cone shaped reference beams form two lines along a diameter at the Fourier planes of the object beam.
Theoretically, for infinite holograms the shift selectivity curve is a sinc(x) like function. See, for example, G. Barbastathis et al.: “Shift multiplexing with spherical reference waves”, Appl. Opt. 35, pp 2403-2417. At the so-called Bragg distances the diffraction efficiencies of the shifted hologram are zero. In WO2006/003077 the distances between the tips of the reference beams along the two lines correspond to these Bragg distances. The interhologram crosstalk between the multiplexed holograms in theory is negligible at the Bragg distances. Assuming infinite diameter holograms there are selective and non-selective directions for the shift multiplexing. See again the article of G. Barbastathis et al. The selective direction is the direction of the line formed by the tips of the reference beams. In the so-called non-selective direction, which is orthogonal to the selective direction in the plane of the holograms, the shift distance is infinite. However, in a real storage system the volume of the hologram is finite. Practically, the order of magnitude of the hologram volume is about (0.4-0.6)×(0.4-0.6)×(0.2-0.6)mm3. Detailed investigations have shown that there are large discrepancies between the shift selectivity curves of infinite and finite holograms. There are no Bragg distances in case of finite volume holograms. See Z. Karpati et al.: “Shift Selectivity Calculation for Finite Volume Holograms with Half-Cone Reference Beams”, Jap. J. Appl. Phys., Vol. 45 (2006), pp 1288-1289. Using finite volume holograms the order of magnitude of the selectivity is similar in both directions. See, for example, Z. Karpati et al.: “Selectivity and tolerance calculations with half-cone reference beam in volume holographic storage”, J. Mod. Opt., Vol. 53 (2006), pp 2067-2088. The presence of selectivity in both directions allows two-dimensional multiplexing. A problem is that the interhologram cross-talk is too high in the non-selective direction. This limits the achievable number of multiplexed holograms in this direction, and as a consequence limits the total capacity of the holographic storage medium.